1. Field of the Invention
The present invention relates to cell designs of a mobile communication system that is based on a CDMA (code division multiple access) scheme typically used in an IS-95-A scheme.
2. Description of the Related Art
As a number of customers increases in a mobile-communication system, there is an increasing need for a system that can accommodate a large number of customers.
FIG. 10 is an illustrative drawing showing a configuration of a typical related-art mobile-communication system.
In the system of FIG. 10, a public telephone network is connected to a mobile network via mobile switch center MSC. The mobile switch center MSC has base station controllers BSC connected thereto, and the base station controllers BSC in turn have base stations BTS connected thereto. Each of the base stations BTS communicates with mobile stations MS residing in its cell (i.e., area of control) so as to render services such as a telephone service. In such a mobile-communication system, a CDMA (code division multiple access) scheme, a TDMA (time division multiple access) scheme, or a FDMA (frequency division multiple access) scheme is typically employed for the purpose of providing multiple accesses.
[CDMA Scheme]
The CDMA scheme is used in the IS-95-A scheme. In the CDMA scheme, a base station uses the same frequency for communicating with different mobile stations residing in its own cell. Channels for communicating with respective mobile stations are established by using predetermined codes, which are called dispersion codes, and serve to discriminate respective signals of mobile stations. Data exchanged between the base station and a mobile station is encrypted (frequency dispersed) by convolving the data with a dispersion code. On the receiver side, the received data is further convolved with the same dispersion code in order to identify a channel.
In the CDMA scheme, a transmitter side of a base station uses two types of dispersion codes. One is a short code, which is used for discriminating the base station from other base stations. The other is a long code, which is used for discriminating a mobile station as a destination. These two codes are convolved with transmission data.
Further, a transmitter side of a mobile station uses two types of dispersion codes. One is a short code again, which is used by a base station for obtaining a data timing of data received from the mobile station. The other is a long code, which serves to discriminate the mobile station from other mobile stations. These two codes are convolved with transmission data.
Such dispersion codes as described above are used for channel-discrimination purposes in the CDMA scheme. Because of this, each mobile station can selectively pick up a channel directed to itself from a relevant base station even when each mobile station simultaneously receives signals of the same radio frequency from a plurality of base stations.
In this manner, the CDMA scheme allows base stations to transmit the same frequency to all the mobile stations, and allows all the mobile stations to transmit the same frequency to the base stations. Please note, however, that the transmission frequency of the base stations is different from the transmission frequency of the mobile stations.
[Hand-off of CDMA Scheme]
“Hand-off” refers to an operation performed when a mobile station moves from a cell of a given base station to a cell of an adjacent base station while engaging in a call. The CDMA scheme performs a soft hand-off operation to insure a continuous call without a break.
During a period of a soft hand-off state, two base stations having bordering cells transmit the same data received from the base-station controller to a mobile station currently positioned around the border of the cells. The mobile station combines the received signals sent from the two base stations, thereby improving a reception gain. Each of the two base stations receives a signal sent from the mobile station, and forwards the signal to the base-station controller. The base-station controller compares the two signals sent from the two respective base stations, and select one having a better signal quality. Selected data is then sent to the mobile-switch center. In this manner, a call never breaks during a soft hand-off period as long as either one of the two base stations securely receives signals from the mobile station.
A mobile-communication system based on the TDMA scheme typically employs a different type of a hand-off operation called a hard hand-off. In a hard hand-off operation, a radio frequency is switched after a mobile station comes sufficiently close to a first base station when moving from a second base station to the first base station with an aim of achieving a secure shift. This means, however, that the mobile station becomes distanced from the second base station before the hand-off operation is actually performed. A hard hand-off thus requires a greater transmission power than a soft hand-off. Further, a communication suffers a brief moment of disconnection at the time of switching.
Even the CDMA scheme may use a hard hand-off operation when two base stations cannot use the same frequency to provide respective services to a mobile station, for example. In such a case, a brief moment of disconnection is observed before a switched channel is reconnected.
[Number of Subscribers in CDMA]
The CDMA scheme achieves division of channels by use of codes, and uses the same radio frequency shared by a large number of mobile stations. When a base station attempts to receive a signal from a given mobile station, other signals transmitted from other mobile stations using the same radio frequency appear to be nothing but sources of interferences for the base station. Namely, an increase in the number of mobile stations adding to the number of transmission signals is tantamount to an increase in noise. The acceptable number of mobile stations that can communicate using the same radio frequency is obviously limited by the degree of interference. It is important, therefore, to reduce interferences by using as small transmission power as possible for each mobile station. This is the most important issue to be addressed in deciding the number of mobile stations than can be accommodated in the same cell, i.e., the number of customers of a single system.
In order for a mobile station to reduce its transmission power around a border of cells, a soft hand-off is suitable because it requires only a minimum transmission power that achieves communication with the closest base station.
As a mobile station shifts its position, a building may come into a line between the mobile station and the base station, or may go out of the line. When the mobile station is obscured by a building, the base station in the CDMA system increases transmission power in response to weakening signals if the CDMA system is not using a soft hand-off. Such an increase in transmission power is an increase of noises as far as other mobile stations are concerned. When the mobile station comes out from behind the building, the transmission power is decreased. Such an adjustment of transmission power is repeated as the mobile station moves.
In a system which employs a soft hand-off, even when a base station is obscured by a building, a mobile station may maintain a connection with another mobile station. In such a case, necessary transmission power is smaller compared to the case of no soft hand-off operation. Namely, a noise effect on other mobile stations is smaller.
Accordingly, a system employing the soft hand-off operation can accommodate a larger number of mobile stations than a system using no soft hand-off, thereby achieving a smaller system cost per user.
[System Configuration of CDMA Scheme]
FIG. 11 is an illustrative drawing showing a configuration of areas (cells) of related-art base stations employing the CDMA scheme.
As previously described, the number of channels that a single base station can use with a common radio frequency is limited by an effect of signal interference. When the number of customers (mobile stations) is larger than the number of channels that can be accommodated by the same frequency, a cell configuration is designed such that a single base station uses different radio frequencies for implementing a plurality of cells. For example, a base station that renders services to more mobile stations than an acceptable number of mobile stations for a single radio frequency needs to implement cells using different radio frequencies.
As shown in FIG. 11, a base station 1 implements a plurality of cells by using a plurality of radio frequencies RF1, RF2, and RF3. Areas covered by the respective radio frequencies RF1, RF2, and RF3 are completely overlapped, and encompass the base station 1 with a radius R1. Further, the areas of the respective radio frequencies RF1, RF2, and RF3 of the base station 1 partially overlap corresponding areas of respective radio frequencies RF1, RF2, and RF3 of a base station 2. This partial overlapping is provided in order to permit a soft hand-off operation between areas using the same radio frequency.
[Selection of Soft Hand-off or Hard Hand-off]
In the CDMA scheme, a decision has be to made as to which one of the soft hand-off and the hard hand-off is used at a border of adjoining cells. To this end, a mobile station obtains the following threshold values from a base station.
1) pilot strength usable for communication
2) pilot strength to trigger hand-off
3) pilot strength lower than the above
A mobile station starts communicating with a base station for location update or the like when finding this base station before any other base stations by picking up a signal from this base station that exceeds “pilot strength usable for communication”.
If a user of the mobile station requests a call, the mobile station sends a call request to the base station. A mobile station constantly searches for pilot channels of surrounding cells, and monitors received strengths of the pilot channels. If any one of the received strengths crosses over from one category to another category classified by the above conditions 1) through 3), the mobile station reports the received strengths of pilot channels to the base-station controller via the currently connected base station.
Based on the reported strengths of pilot channels of surrounding cells, the base-station controller selects one of the following operations.
1) soft hand-off
2) hard hand-off
3) maintain current state
If a soft hand-off or a hard hand-off is selected, a hand-off switch message is sent to the mobile station, thereby prompting the mobile station to switch over to one of the surrounding cells.
In this process, a decision as to which one of the two hand-off operations is selected is made by taking into account the following factors.
1) Soft Hand-off
Conditions that must be satisfied in order to select a soft hand-off are as follows:                a received pilot strength of a surrounding cell that is reported by the mobile station exceeds “pilot strength usable for communication”; and        a target cell (a surrounding cell that is currently evaluated) has an available resource for the same frequency and the same frame offset as those of the currently used cell.        
Such a soft hand-off achieves a switch to the target cell using the same radio frequency and the same frame offset as those of the currently used cell.
In the example of FIG. 11, each of the base stations 1 and 2 uses the radio frequencies RF1, RF2, and RF3 to communicate with mobile stations. Even though a plurality of the radio frequencies RF1, RF2, and RF3 are used, overlapping is provided between the cells using the same frequency. A soft hand-off thus can be performed for a mobile station 3 between the cells using the same frequency.
The frame offset refers to a position in a series of time slots that are used for exchanging communication signals of mobile stations between a base station and a base-station controller on a communication line utilizing a time-division multiplex scheme. A soft hand-off can not be performed unless a position of a time slot of a mobile station is the same in a base station after a hand-off as was in a base station before the hand-off. Therefore, a check has to be made as to whether a frame offset (i.e., a particular time slot) used in a base station before a hand-off is available in a base station to be used after the hand-off. That is, whether the same frame offset is available in the base station to be used needs to be checked in order to perform a soft hand-off operation.
2) Hard Hand-off
Conditions that must be satisfied in order to select a hard hand-off are as follows:                a received pilot strength of a surrounding cell that is reported by the mobile station exceeds “pilot strength to trigger hand-off”;        a pilot strength of a currently used cell is below “pilot strength usable for communication”;        a target cell has available resources; and        the target cell does not have an available space for the same frequency and the same frame offset as those of the currently used cell.        
A hard hand-off may include a case where a switch is made to a different radio frequency when moving into a target cell or a case where a switch is made to a different frame offset while using the same radio frequency.
3) Maintaining a Current Status
Conditions that must be satisfied in order to maintain a current status are as follows.                a received pilot strength of a surrounding cell that is reported by the mobile station exceeds “pilot strength to trigger hand-off”.        a pilot strength of a currently used cell is above “pilot strength usable for communication”; and        a target cell has no available resources, or does not have an available space for the same frequency and the same frame offset as those of the currently used cell.        
When a decision is made to keep a current status, no hand-off is performed, and a connection with the current base station remains as it is.
In this manner, a hand-off operation is performed by evaluating received pilot strengths that are reported to a base-station controller from a mobile station. Decisions as to whether to perform a hand-off operation and which type of hand-off operation is to be performed are made by the base-station controller. To this end, the base-station controller needs to keep track of locations of and frequencies used by all the mobile stations.
[Details of Hard Hand-off in CDMA]
FIG. 12 is an illustrative drawing showing a hard hand-off operation performed by a mobile station.
In FIG. 12, the base stations 1 and 2 are under the control of a base-station controller 4. Ellipses drawn above the base stations 1 and 2 illustrate cells (areas) covered by the radio frequencies RF1 and RF2. Points a through f indicate positions of the mobile station 3. What is shown in the middle of the figure demonstrates pilot strengths of the base stations 1 and 2 that are received by the mobile station 3 as it moves along. In this presentation, a pilot strength x indicates a “pilot strength usable for communication”, and a pilot strength y indicates a “pilot strength to trigger a hand off”.
In the following, a series of operations from when the mobile station 3 starts communication with the base station 1 at the point a by using the radio frequency RF1 to when the mobile station 3 finally reaches the point f will be described.
When the mobile station 3 reaches the point c, the received pilot strength of the base station 2 exceeds the pilot strength y (i.e., “pilot strength to trigger a hand-off”). The mobile station 3 reports this change to the base-station controller 4 via the base station 1.
The base-station controller 4 makes a resource request to the base station 2 with an aim of performing a soft hand-off operation. In this example, however, there is no resources, and a current status is maintained.
When the mobile station 3 moves to the point e, the received pilot strength of the base station 1 becomes smaller than the pilot strength x (i.e., “pilot strength usable for communication). The mobile station 3 reports this to the base-station controller 4 via the base station 1. The base-station controller 4 instructs the base station 1, the base station 2, and the mobile station 3 to carry out a hard hand-off operation. The hard hand-off operation is carried out at the point e. In this manner, the mobile station 3 communicates with the base station 1 from the point a to the point e, and communicates with the base station 2 from the point e to the point f.
[Details of Soft Hand-off in CDMA]
FIG. 13 is an illustrative drawing showing a soft hand-off operation performed by a mobile station. In FIG. 13, the same numerals and symbols as those of FIG. 12 are used for referring to the same items.
In the following, a series of operations from when the mobile station 3 starts communication with the base station 1 at the point a by using the radio frequency RF1 to when the mobile station 3 finally reaches the point f will be described.
When the mobile station 3 reaches the point c, the received pilot strength of the base station 2 exceeds the pilot strength y (i.e., “pilot strength to trigger a hand-off”). The mobile station 3 reports this change to the base-station controller 4 via the base station 1. The base-station controller 4 makes a resource request to the base station 2 with an aim of performing a soft hand-off operation. When resources are secured, the base-station controller 4 instructs the base stations 1 and 2 and the mobile station 3 to carry out a soft hand-off operation, so that the mobile station 3 starts communicating with both of the base stations 1 and 2.
When the mobile station 3 moves to the point e, the received pilot strength of the base station 1 becomes smaller than the pilot strength x (i.e., “pilot strength usable for communication). The mobile station 3 reports this to the base-station controller 4 via the base stations 1 and 2. The base-station controller 4 instructs the base station 1, the base station 2, and the mobile station 3 to end the soft hand-off operation. As a result, the mobile station 3 communicates only with the base station 2. In this manner, the mobile station 3 communicates with the base station 1 from the point a to the point e, and communicates with the base station 2 from the point c to the point f. Between the point c and the point e, the soft hand-off operation is being engaged, allowing simultaneous communications with the two base stations.
[Configuration of Base Station and Base-Station Controller]
FIG. 14 is a block diagram showing a related-art configuration of a base station and a base-station controller.
The base station includes a plurality of identical configurations as many as there are used radio frequencies (i.e., three in this example since three radio frequencies RF1, RF2, and RF3 are used).
The base station is provided with two antennas with respect to each radio frequency for signal exchanges with mobile stations. One antenna is used for transmission of signals, and the other antenna is used for receiving signals.
On a receiver side, RF-conversion units 301 through 303 convert a radio signal received by the antenna into an intermediate frequency signal, which is then demodulated by a QPSK-modulation/demodulation unit 31 before being sent to CDMA-modulation/demodulation units 320 through 32n. The CDMA-modulation/demodulation units 320 through 32n are provided as many as there are mobile stations that can communicate simultaneously with the base station. In this example, therefore, the base station can establish simultaneous communications with n+1 mobile stations. The CDMA-modulation/demodulation units 320 through 32n convolve the received signals with dispersion codes so as to attend to an inverse-dispersion process of the CDMA signals. The dispersion codes are determined by a BTS-control unit 33 in advance. A BSC-connection unit 34 receives the received signals having the inverse-dispersion process applied thereto, and forwards the them to the base-station controller.
On a transmitter side, the BSC-connection unit 34 receives transmission data from the base-station controller, and sends it to one of the CDMA-modulation/demodulation units 320 through 32n selected in advance by the BTS-control unit 33. The selected one of the CDMA-modulation/demodulation units 320 through 32n convolves the transmission data with a dispersion code to attend to a CDMA-dispersion process. Further, the QPSK-modulation/demodulation unit 31 applies a QPSK modulation to generate an intermediate frequency signal. One of the RF-conversion units 301 through 303 converts the intermediate signal into a radio transmission signal, and transmits it via the antenna.
FIG. 15 is a block diagram of a RF-conversion unit 30 of the base station. The RF-conversion unit 30 is any one of the RF-conversion units 301 through 303.
On the receiver side of the RF-conversion unit 30, a band-pass filter 301 filters a received radio signal, and, then, a low-noise amplifier 302 amplifies the filtered signal. A multiplier 303 multiplies the amplified signal by an output of a receiver local-signal generator 306-1 to obtain an intermediate frequency signal.
On a transmitter side of the RF-conversion unit 30, an intermediate frequency signal is filtered by a band-pass filter 304. A multiplier 305 multiplies the filtered signal by an output of a transmitter local-signal generator 306-2 to generate a radio transmission signal. The radios transmission signal is amplified by a high-power amplifier 308, and, then, is transmitted from the antenna.
With reference to FIG. 14 again, on a receiver side of the base-station controller, data sent from a plurality of base stations are received by a BTS-connection unit 11, and are provided to a communication setting unit 12. The communication setting unit 12 supplies the received data to corresponding selection units 130 through 13m as a given chunk of the received data has an allocated selection unit. This allocation is determined by a BSC-control unit 16. Each of the selection units 130 through 13m selects one of the two received data chunks that has fewer errors than the other during a period of a soft hand-off operation, and, then, applies an audio-decoding process before sending the selected data to a MSC-connection unit 15. The MSC-connection unit 15 combines data supplied from the selection units 130 through 13m to generate frames, and sends these frames to a mobile-switch center 5.
On a transmitter side of the base-station controller, frames received from the mobile-switch center 5 are processed to extract transmission data, which is then sent to one of the selection units 130 through 13m that is preselected by the BSC-control unit 16. The one of the selection units 130 through 13m applies an audio-coding process before sending the transmission data to the communication setting unit 12. The transmission data is then transmitted via the BTS-connection unit 11 to a destination that is specified by the BSC-control unit 16.
[Configuration of Selection Unit]
FIG. 16 is a block diagram of a selection unit of the base-station controller. The selection unit 13 of FIG. 16 is any one of the selection units 130 through 13m.
The selection unit 13 includes a first buffer 131, a second buffer 132, a third buffer 133, an audio decoding unit 134, an audio coding unit 135, a buffer-control unit 136, a demultiplexer 137, a first check unit 138, a second check unit 139, and a selector 140.
On a receiver side of the selection unit 13, the demultiplexer 137 receives data, and supplies a first one of two data chunks consecutively received to the first check unit 138 and a second one of the two data chunks to the second check unit 139 if a soft hand-off operation is being engaged. The buffer-control unit 136 is notified when the data transfer is completed. The first check unit 138 and the second check unit 139 check errors in the received data, and send the received data to the first buffer 131 and the second buffer 132, respectively. Results of the error checks are provided to the buffer-control unit 136. The buffer-control unit 136 controls the selector 140 to select one of the two data chunks that has the smallest errors, and controls a corresponding one of the first buffer 131 and the second buffer 132 to supply the received data to the audio decoding unit 134. These operations as described above are repeated for each frame. If the soft hand-off operation is not being engaged, the demultiplexer 137 supplies data to the first check unit 138 as it receives the data.
On a transmitter side of the selection unit 13, the audio coding unit 135 applies audio-coding processing to transmission data, and sends the processed transmission data to the third buffer 133. Under the control of the buffer-control unit 136, the third buffer 133 supplies the transmission data to the communication setting unit 12.
[Configuration of Mobile Station]
FIG. 17 is a block diagram of a receiver portion of a related-art mobile station.
The mobile station of FIG. 17 includes a RF-conversion unit 21, a QPSK-demodulation unit 22, a searcher 23, a finger-control unit 24, a first finger 25, a second finger 26, a control unit 27, a maximum-ratio-integration unit 28, a signal processing unit 29, and an audio decoding unit 210.
A signal received at the antenna is supplied to the RF-conversion unit 21, where the received signal is changed into an intermediate frequency signal. The intermediate frequency signal is demodulated by the QPSK-demodulation unit 22, and, then, the demodulated signal is provided to the searcher 23, the first finger 25, and the second finger 26.
The searcher 23 includes a searcher-control unit 231, a correlation unit 233, a peak-detection unit 234, and a timing-generation unit 235. The searcher-control unit 231 indicates a dispersion code to be searched for and a time span during which the search is to be conducted. The correlation unit 233 detects a correlation between a pilot signal of a currently used base station and a pilot signal of a surrounding base station as these pilot signals are contained in the demodulated received signals. The peak-detection unit 234 detects a peak in an output of the correlation unit 233, and the timing-generation unit 235 generates a timing signal indicative of a timing of the peak. The timing signal is supplied to the finger-control unit 24. The finger-control unit 24 obtains a delay profile of the received signal of the currently used base station by using the timings reported from the searcher 23. In a descending order of the correlation in the delay profile, the finger-control unit 24 notifies the first finger 25 and the second finger 26. Further, the finger-control unit 24 reports the received pilot strength of the surrounding base station to the control unit 27.
The first finger 25 and the second finger 26 have the same configuration. Each finger includes a timing-synchronization unit 251, a correlation unit 252, and a correlation-value detecting unit 253. The correlation unit 252 calculates a correlation between the received signal and the dispersion code that is specified by the control unit 27 in advance. The correlation-value detecting unit 253 detects a correlation value at a timing specified by the timing-synchronization unit 251, and the detected correlation value is supplied to the maximum-ratio-integration unit 28. The maximum-ratio-integration unit 28 attends to a maximum-ratio-integration process with respect to the correlation values supplied from the first and second fingers 25 and 26, and supplies the integrated signal to the signal processing unit 29. The signal processing unit 29 attends to error corrections, and the audio decoding unit 210 reproduces audio from the error-corrected signals. Here, if the data output from the signal processing unit 29 is a control message, the control message is supplied to the control unit 27.
In the related-art configuration as described above, a single station may have a plurality of cells using a plurality of radio frequencies RF1, RF2, and RF3 as shown in FIG. 11. In such a case, a soft hand-off can take place in any one of the radio frequencies RF1, RF2, and RF3, and, thus, hardware and software for providing a soft hand-off function are required with respect to each radio frequency. Namely, every single one of the selection units 130 through 13m of the base-station controller needs to have a function to select one of the two received data sets in order to achieve a soft-hand-off operation. This results in an undesirable cost increase.
Between adjacent base stations, areas covered by the same radio frequency are overlapped at a peripheral portion. When a hard hand-off operation is engaged because a soft hand-off is not available due to lack of resources, the mobile station 3 may move deep into a new cell to arrive at the point e while keeping communication with the base station of an old cell. In such a case, signals transmitted from the base station 2 appear to be nothing but noises to the mobile station 3. Further, the transmission signals of the base station 2 are stronger than transmission signals coming from the base station 1 that is currently used. Namely, the signals transmitted from the base station 2 interferes with communications of the mobile station 3 residing within the cell of the base station 1. These factors further limits the number of mobile stations that can be used in the system.
As shown in FIG. 12, when the mobile station 3 having a connection with the base station 1 is located at the point e, the mobile station 3 needs to transmit signals with such a strong power as to make them reach the base station 1 by covering the distance r1. As far as the base station 2 located only a distance r5 from the mobile station 3 is concerned, such strong transmission from the mobile station 3 at the point e is a source of interference against signals coming from other mobile stations. This factor further limits the number of mobile stations that can be used in the system.
In the related art, a soft hand-off operation should be usable regardless of what radio frequency is used by a mobile station. In this configuration, a mobile station shifting a position thereof may come close to a neighboring base station, resulting in a change in a received pilot strength. Because of this, a delay profile needs to be constantly monitored for all the radio frequencies with respect to the neighboring base stations in addition to a delay profile of multi-path components. As a result, it is necessary to keep the searcher 23 in operation all the time for monitoring purposes. This can be achieved, however, at a cost of an increase in power consumption.
Accordingly, there is a need for a CDMA mobile communication system which can accommodate a large number of mobile stations at a low cost while providing soft hand-off services to the mobile stations.